The invention relates to a photoelectric detection device for radiation having wavelengths in a given range of the spectrum. The device comprises, in an evacuated envelope, a photosensitive layer which is provided on a substrate which is transparent to incident radiation. The device further comprises an intermediate layer for optical adaptation. The intermediate layer is also transparent to the incident radiation, and it is provided between the photosensitive layer and the substrate. The refractive index of the material of the intermediate layer is between that of the substrate and that of the material of the photosensitive layer. Such devices may be, for example, photoelectric cells, image intensifier tubes, display tubes integrated in television pick-up systems, or photomultipliers.
When a photoelectric detection device comprises a photosensitive layer provided directly on a substrate, this results in general in a poor optical adaptation of the photosensitive layer to the substrate. As a result, a large part of the light incident on the substrate is not effectively used for the conversion of photons into electrons. The photoelectric detection efficiency of the device is thereby considerably reduced. It is known to improve the efficiency of such a device by attenuating the reflections formed at the interface between the substrate and the photosensitive layer. The reflections are attenuated by means of one or more intermediate layers which are transparent to the incident radiation and which are disposed between the substrate and the photosensitive layer.
Such a device having only one single intermediate layer is known, for example, from U.S. Pat. No. 3,254,253 (Davis, et al). The intermediate layer in this case has been chosen for its weak absorption. Moreover, the optical constants and the thickness of the intermediate layer are chosen such that, taking into account the optical constants of the substrate and of the photosensitive layer, the reflected light rays at the interface between the substrate and the intermediate layer and the reflected rays at the interface between the intermediate layer and the photosensitive layer, respectively, have exactly the same amplitude and opposite phases so that they neutralize each other by interference.
Such a device attenuates the losses as a result of reflections to a considerable extent but does not necessarily result in a device having an optimum efficiency.